cmd/internal/obj/loong64: add ADDU16I.D instruction support

cmd/internal/obj/loong64: add ADDU16I.D instruction support

Go asm syntax:
	ADDV16		$(1<<16), R4, R5

Equivalent platform assembler syntax:
	addu16i.d	r5, r4, $1

Change-Id: Ica4a4e779d0a107cda3eade86027abd6458779a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/699056
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>